The present invention relates to a magnetic radial bearing having a stator which has a first coil, a second coil, a third coil and a fourth coil, the first coil and the third coil thereof on a first axis opposing the second coil and the fourth coil thereof on a second axis. The present invention further relates to a method for controlling such a magnetic radial bearing.
Magnetic radial bearings produce eddy currents in rotating shafts. Said eddy currents lead to undesired heating of the shaft. The power loss associated therewith reduces the efficiency of the machine. A laminated magnetic return path on the shaft, which would reduce the eddy currents, decreases the rigidity of the shaft.
Hitherto, therefore, a compromise was sought between the lowest possible eddy current losses and the greatest possible rigidity. The pole numbers of the magnetic fields, the rotational speed of the shaft and the type of lamination are substantially responsible for the eddy current losses. In order to achieve low magnetic reversal frequencies, a low pole number is sought. As a result, however, the magnetic field penetrates deep into the rotor and thus requires a lamination with a deep rotor armature, resulting in a thin shaft. If a critical level of inherent instability is exceeded, the pole number has to be increased, which in turn leads to higher frequencies and losses.
A cross-section through a conventional radial magnetic bearing is shown in FIG. 1. The stator in this case has eight coils inserted in axial grooves, wherein in FIG. 1 only the coils 1 and the coil cores 2 are indicated schematically. A rotor 3 is magnetically retained in the interior of the stator. The rotor 3 is the shaft to be mounted. The axes of the coils 1 in this case run substantially radially relative to the rotational axis of the rotor 3.
Radial magnetic bearings with axial coils are also known from the book “Magnetic Bearings” by Gerhard Schweitzer and Eric H. Maslen, Springer Verlag Berlin, 2009, XV, Pages 82 to 84 and 96. In other words, the coil axes extend parallel to the bearing axis. Accordingly, the flux is guided substantially in the axial direction both in the coils and in the rotor.